The present invention relates to a method and apparatus for epitaxially growing a Group III-V compound semiconductor on a monocrystalline substrate.
As a method for epitaxially growing a Group III-V compound semiconductor on a monocrystalline substrate, a vapor phase epitaxy (VPE) method, a metal organic chemical vapor deposition (MOCVD) method, and a molecular beam epitaxy (MBE) method are known. In a hydride VPE method, a Group III material is converted to a halogenide, and is supplied into a reaction tube together with H.sub.2 diluted gas containing a Group V element so as to grow a Group III-V compound semiconductor on a substrate. By this method, in the case of homo-epitaxial growth wherein a crystal is grown on a substrate of the same type material, a crystalline film having a high purity and good crystallinity can be easily obtained. Therefore, this method is widely applied to the manufacture of a Group III-V compound (e.g., GaAs, GaP, InP, and the like) semiconductor device.
The conventional hydride VPE method in the case of GaP growth will now be described with reference to FIGS. 11(A) and 11(B). Ga 3 of a Group III material stored in a boat 2 and a GaP monocrystalline substrate 4 are arranged inside a reaction tube 1. An electrical oven (not shown) is arranged around the reaction tube 1, so as to keep the temperature distribution inside the reaction tube, as shown in FIG. 11(B). A gas mixture 6 containing PH.sub.3 and H.sub.2 is introduced from an introduction tube 5 inside the reaction tube and a gas mixture 7 containing HCl and H.sub.2 is introduced through a gas inlet (not shown). The introduced HCl reacts with the heated Ga. EQU Ga+HCl.fwdarw.GaCl+1/2H.sub.2 ( 1)
A gas mixture 8 containing H.sub.2 and GaCl produced by the reaction given by reaction formula (1) is supplied toward downstream side. PH.sub.3 is decomposed into P.sub.4, and is mixed with the gas mixture 8 containing GaCl and H.sub.2 in a baffle 9 placed on a high-temperature area, thereby forming a gas mixture 10 containing P.sub.4, GaCl, and H.sub.2. The gas mixture 10 passes by the high-temperature region and then flows into the low-temperature region. As the temperature of gas mixture 10 is reduced, GaP grows on the GaP monocrystalline substrate 4 placed on the low-temperature region by the reaction expressed by reaction formula (2). EQU 2GaCl+1/2P.sub.4 +H.sub.2 .fwdarw.2GaP+2HCl (2)
In the case of homo-epitaxial growth wherein GaP is to be grown on the substrate 4 of a GaP single crystal, GaP can be grown epitaxially as described above. When an Si single crystal is used as the substrate 4, GaP cannot be grown for the following reasons. The reaction tube is normally made of silica glass (SiO.sub.2) because of its good heat resistance and machinability. When HCl is brought into contact with the silica glass which is heated to high temperature, H.sub.2 O is produced by the following reaction: EQU SiO.sub.2 +(4-n)HCl+nH.sub.2 .fwdarw.SiHnCl.sub.4-n +2H.sub.2 O (3)
(where n=0 to 3)
H.sub.2 O is supplied above the Si substrate on the downstream side together with the H.sub.2 gas and reacts with Si, thereby producing SiO.sub.2. The thus produced SiO.sub.2 covers the Si substrate surface and prevents from GaP growth on the Si substrate.
The article of H. Huber and G.H. Winstel (Siemens Forschungs und Entwichlungsberichte; 2 (1973) pp. 171-174) describes an example wherein an electrical oven for annealing is provided in addition to the growth electrical oven, the Si substrate is annealed in the H.sub.2 atmosphere to remove a natural oxide film, and immediately thereafter, the substrate is moved to a growth region so as to grow GaP. However, in this case, since the substrate is exposed to the HCl atmosphere at high temperature of about 800.degree. C., formation of SiO.sub.2 on the Si substrate cannot be avoided. For this reason, growth reproducibility is poor, and even if grown, the resultant crystal has a poor crystallinity and cannot be used as a substrate of practical device.
For the above reasons, the hetero-epitaxial growth of the Group III-V compound semiconductor on the Si substrate has not yet been realized by the epitaxial growth method containing halogenide such as HCl.
There are several attempts to grow Group III-V compound semiconductor on Si substrate by the MOCVD or MBE. However, these methods cannot provide a crystal having crystallinity comparable to that of a crystal which is homo-epitaxially grown by the epitaxial growth method containing a halogenide.
M. Akiyama et al. reported a GaAs crystal which is grown on an Si substrate by the MOCVD method (Journal of Crystal Growth, Vol. 68, pp. 21-26, 1984). With this report, a GaAs film which was deposited on the Si substrate by decomposing trimethyl gallium and AsH.sub.3 at 400.degree. C. was annealed at 750.degree. C., and mirror-surface like monocrystalline GaAs could be obtained. However, even when GaAs was grown on the resultant GaAs film by a conventional method, such as the MOCVD or hydride VPE method, the resultant crystal could not provide the same crystallinity as that of a crystal which was grown on a GaAs substrate. It was revealed from impurity analysis in the depth direction of the resultant film that CH.sub.3 as a constituting molecule of trimethyl gallium is left in the film without being decomposed and degrades the crystallinity since a first film is deposited at a low temperature of 400.degree. C.
Attempt for developing a device for integrating many or various functions on an identical substrate has been made. For example, an optical-electronic device in which optical devices and electronic devices are integrated on an identical substrate is known. When devices made of different materials are integrated on an identical substrate, in order to fabricate the next device so as not to impair the previously fabricated device, the steps for growing the respective materials must be performed at low temperature. However, in the conventional method, when growth is carried out under the condition of decreased substrate temperature, a crystal is deposited on a region of the inner wall of the reaction tube corresponding to a temperature, at which the growth is conventionally performed, on the upstream side of the substrate position, and no crystal is grown on the substrate. In this manner, hetero-epitaxial growth for integrating many or various functions on an identical substrate cannot be grown by the conventional hydride VPE method.